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The nervous system
The nervous system allows the body to respond to changes in
the environment. This is a process usually coordinated by the
brain. Reflex actions are extrarapid responses to stimuli
[stimuli: things that set off a reaction in the nervous system for
example, light, heat, sound, gravity, smell,…

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nerve cells neurones to the brain. The brain then coordinates the

An effector is any part of the body that produces the response. Here
are some examples of effectors:
a muscle contracting to move the arm
a muscle squeezing saliva from the salivary gland
a gland releasing a…

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A motor neurone

Where two neurones meet, there is a tiny gap called a synapse. Signals
cross this gap using chemicals. One neurone releases the chemical into
the gap. The chemical diffuses across the gap and makes the next
neurone transmit an electrical signal.

Reflex actions
When a receptor…

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The way the iris in our eye adjusts the size of the pupil in response to
bright or dim light is also a reflex action.
In bright light:
Radial muscles of the iris relax.
Circular muscles of the iris contract.

Less light enters the eye through the contracted pupil.



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